HEAT. 273 



encountered on the way up ; and the same may be true of the iron, the reduction in this 

 case having been effected, as J. L. Smitli suggested, by the aid of some carbon compound 

 in the ascending liquid basalt. But it may be that the iron was carried up by the liquid 

 rock from the earth's interior. 



Some igneous rocks consist chiefly of the " infusible " minerals chrysolite and leucite ; 

 but the complete fusion which the capability of flowing indicates is evidence that some 

 part of the constituents of these rocks before ejection were in fusible combinations. By 

 infusible is here meant infusible before the common blowpipe. 



The more important volcanic phenomena connected with these rocks 

 depend on the temperature of fusion, those requiring the least heat being 

 the earliest to fuse as the temperature rose, and the longest to continue 

 liquid as it declined, and, therefore, those that have commonly had, when 

 ejected, the temperature of the freest liquidity. 



There are three prominent classes of igneous rocks, differing in fusibility. 

 In each class the kinds are nearly alike in chemical constitution, but differ 

 somewhat mineralogically and in state of crystallization. There are inter- 

 mediate kinds ; but still the classes stand out prominently. These three 

 groups are a,s follows : — 



1. Easy fusibility. — The Basaltic class : These fuse at about 2250° F. 

 (C. Barus); consist chiefly of pyroxene (or a related species), and of the 

 feldspar, labradorite, whose alkalies are lime and soda; they often carry 

 grains of chrysolite, but very rarely of quartz : as basalt, doleryte, diabase, 

 gabbro, etc. These rocks are basic (pages 65, 86); but fusibility, not basicity, 

 is the important characteristic as regards volcanic phenomena ; for anorthite, 

 the most basic of the feldspars, is one of the most infusible. 



2. Medium fusibility (about 2520° F., Barus). — The Andestte class: 

 These consist of a mineral of the pyroxene-hornblende group, and, as the 

 feldspar portion, of oligoclase or andesite, whose alkalies are soda and lime ; 

 they often carry quartz grains : as andesyte, dacyte, quartz-andesyte, dioryte, 

 and related kinds. 



3. Difficult fusibility (about 2700° F., Barus, for quartz-trachyte or rhyo- 

 lyte). — The Trachyte class: These consist of potash-feldspar, orthoclase, 

 or of orthoclase with a little oligoclase, or albite ; sometimes containing mica, 

 pyroxene, hornblende, quartz : as trachyte, rhyolyte, f elsyte, granite, etc. 

 Ehyolyte is quite viscid even at 3100° F. (Barus). 



Lavas, especially the trachytic and andesytic kinds, and including lithoid obsidian, 

 have frequently a thin laminated structure, which is produced, not by a succession in 

 streams, the laminae being too thin for streams, but by successive action in the supply of 

 lava at the point of outflow ; the incipient subdivisions are drawn out as the stream 

 flows into thin sheets or layers (Iddings) . 



Fouque and Levy obtained from fused basalt on cooling after being for 48 hours at 

 white-red fvision, " a temperature above the melting-point of pyroxene and labradorite," 

 "crystals of olivine in a brownish vitreous magma"; but on cooling from cherry-red fusion 

 sustained for 48 hours, numerous microlites of labradorite and pyroxene with magnetite. 



Messrs. Ch. and G. Friedel (1890), on heating mica to 500° C. (900° F.) with alkaline 

 dana!s manual — 18 



